1. Field of the Invention
This invention relates to auto transmissions having a layshaft kinematic arrangement, particularly to such transmissions having input clutches but no torque converter.
2. Description of the Prior Art
Automatic transmissions for transmitting power between an input and an output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open friction clutches and brakes, hydraulic pump, and gear meshes.
To improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce all of these parasitic losses except gear mesh losses. An ASM transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next speed ratio by actuating couplers, and then restoring torque. A primary functional feature of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change because the couplers cannot be actuated while transmitting power.
Dual clutch layshaft transmissions are essentially two ASM transmissions, one providing odd numbered gears and one providing even numbered gears. Shifts between odd numbered gears and even numbered gears can be accomplished without interrupting power flow. While operating in an odd gear, the couplers can be actuated to configure the transmission for the next even gear. Dual clutch transmissions have parasitic losses only slightly higher than ASM transmissions.
Layshaft automatic transmissions offer significant efficiency improvements over conventional step-change automatic transmissions with torque converters. Especially when applied to trucks, however, they must produce more torque multiplication than would be required of a transmission having a torque converter to avoid dissipating excessive energy in the clutch during launch of the vehicle from a stopped condition. This requirement for greater torque multiplication in the gearbox also compensates for the torque multiplication that a torque converter produces at lower speeds.
In order to share energy between the dual start-up clutches during launch, it is desirable to have small torque ratio steps between the launch gear ratio and the next higher gear ratio. But providing a large torque ratio span with relatively small ratio steps usually requires a large number of gears and couplers. It is preferable to minimize the number of gears and couplers to reduce package space and to lower cost.